panfrost: Fix infinite growth of the BO access readers array

Similar to 5e8386c6 ("panfrost: Compact the bo_access readers
array"), rebuild the array to remove the holes before appending to it.

Fixes: a6af15aa ("panfrost: Simplify the dependency tracking logic")
56 jobs for !11442 with readers-leak in 10 seconds (queued for 3 seconds)
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